In general, a brake system of a vehicle is operated to slow down or stop the vehicle that is in travel.
Brake systems are largely classified into a hydraulic brake that brakes a vehicle using hydraulic pressure generated by operating a brake pedal, a hydraulic servo brake that is designed to be used for vehicles requiring a large brake force, such as large-sized vehicles, and an air brake that brakes a vehicle using pressure of compressed air.
In the hydraulic brake system, a master cylinder generates hydraulic pressure using a booster that increases the operational force of the brake pedal and supplies the pressure to a caliper on the wheel or a wheel cylinder in the drum, and this operation is very important, that is, the performance of the master cylinder is the main factor to determine the performance of the brake system.
A plunger-typed master cylinder of master cylinders in the related art includes a primary piston that receives a force from a booster increasing a stepping force applied on the brake pedal and a floating piston that is disposed in the cylinder and actuated by the force transmitted to the primary piston, in which hydraulic pressure is generated by the primary piston and the floating piston in the master cylinder, respectively.
The plunger-typed master cylinder is commonly provided with seal cups with the inner circumferences being in contact with the primary piston and the floating piston and the outer circumferences with the insides of a hydraulic pressure chambers to generate hydraulic pressure, such that the seal cups function as oil passages while the brake is not in operation or the stepping force is released and stop the oil flow in braking.
However, the contact conditions of the inner and outer circumferences of the seal cups inserted in the hydraulic pressure chambers in the master cylinder are different, that is, the inner circumferences of the seal cups are deformed because it is in contact with the pistons that move in operation of the master cylinder, whereas the outer circumferences are kept in contact with the insides of the hydraulic pressure chambers, such that deformation is different at the inner and outer circumferences. Therefore, when the difference in deformation of the seal cups is excessive, it may reduce the durability due to increase in the contact areas, in addition to reducing the oil supply passage.